A wideband Software-Defined Radio (SDR) receiver is a promising solution to satisfy the demand for the multi-standard and multi-band wireless system. Most SDR receivers use a direct conversion architecture to obtain high integration. Telecommunication standards such as GSM, WCDMA, LTE, WLAN, etc., define in their specifications strict requirements for suppression of undesired signals. To comply with the requirements a Surface Acoustic Wave (SAW) filter may be used for each supported standard. However, driven by the market requirements for smaller and power efficient devices to decrease the Bill-of-Material (BoM) of the wireless devices, the SAW filters may be removed. This can cause problems that detrimentally affect the receiver performance. A direct-conversion receiver lacking a SAW filter may require more stringent constraints on some of the non-idealities of its analog circuitry, affecting the overall receiver performance.
In a transceiver including a transmitter and a receiver, a signal transmitted by the transmitter leaking into the front-end path of the receiver can detrimentally affect its performance. For example, in a receiver with no SAW filter and only a duplexer isolating the receiver from the transmitter, non-linearity problems may arise due to the considerably reduced attenuation of the leaked transmitted frequency into the receiver path. As the unwanted signals—interferences or leaked transmitter signal—are not suppressed, the receiver sensitivity may be degraded by the second order intermodulation distortion (IMD2) caused by its non-linear elements. In fact, every signal, even the desired signal, entering a non-linear element gives rise to distortion products by generating a signal centred at zero frequency (DC frequency). A highly linear receiver is thus desired to help to prevent generating distortion products at baseband frequency.
The second order intercept point (IP2) is a measure of the second order non-linearity of the receiver and helps quantifying the input level at which the power of second order intermodulation distortion (IMD2) products at baseband frequency equals the power of the desired signal. The presence of second order intermodulation distortion (IMD2) products, generated when a non-linear receiver is exposed to a multi-tone continuous wave signal or to an amplitude modulated signal, can substantially reduce the receiver sensitivity. A major source of non-linearity affecting the IP2 performance of a receiver includes transistor mismatch and layout asymmetry in the mixer. Thus, without a high IP2 of the mixer, the receiver sensitivity is greatly reduced.